Inspect the rafters. Measure the distance between the joists. Joists farther than 16 inches apart can't carry as much weight as joists that are 16 inches apart. Measure the distance from the top of the joists to the top of the garage ceiling so you know what will fit in the rafters. Don't stack items on wires or cables. Keep all stored items away from air vents.
Cut sheets of plywood into manageable sizes and lay them across the joists. Create decking with the plywood sheets so the stored items have a solid surface to sit on. Decide how often you'll need to access the stored items and place the plywood accordingly. For items you need on a regular basis, create decking in a U shape around the parameter. For items you rarely need, deck the back half of the garage rafters.
Purchase plastic containers with lids that close securely to prevent insect infestation or water intrusion. Choose containers designed to fit the items you need to store. Find containers with easy to access handles to help you retrieve the stored items easily. Look for nesting containers so stacked containers are less apt to tip or fall. Label each container or use clear containers so you can easily see the contents.
Spread the weight of heavy objects over at least two rafters. Store long or bulky things spread out over several rafters. Make sure nothing but very light weight items or containers are sitting only on the plywood decking. Be aware of weight when adding storage or retrieving items from storage. Don't put your body weight on the plywood decking.